The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, three-piece or multi-layered golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded thereon. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance especially in terms of the distance such balls travel and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer a so called “three-piece ball” results and similarly, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Wound balls typically have either a solid rubber, or liquid-filled, center around which many yards of a stretched elastic thread or yarn is wound to form a core. The wound core then is covered with a durable cover material, e.g., an ionomer or other thermoplastic material or a softer cover such as balata or cast polyurethane. Wound balls generally are softer than two-piece balls, and they provide more spin, which enables a skilled golfer to have more control over the ball's flight. In particular, it is desirable for the golfer to be able to impart backspin to the ball, for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial backspin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart backspin onto a golf ball is related to the extent to which the golf ball's cover deforms when it is struck by a golf club. Because conventional wound balls are generally more deformable than are conventional two-piece balls, it is easier to impart spin to wound balls. However, higher spinning wound balls typically travel a shorter distance when struck, as compared to two-piece balls. Moreover, because wound balls generally have a more complex structure, they generally require a longer time to manufacture and are more expensive to produce than are two-piece balls.
Golf balls having a two-piece construction generally are most popular with the recreational golfer, because they are relatively durable and provide maximum distance. Two-piece balls have a single solid core, usually formed of a cross-linked rubber, which is encased by a cover. Typically, the solid core is made of polybutadiene, which is chemically cross-linked with peroxide, or sulfur compounds together with co-cross-linking agent, such as zinc diacrylate. The cover of such balls often comprises tough, cut-proof blends of one or more materials known as ionomers, which typically are ethylene/acrylic acid copolymers or ethylene/acrylic acid/acrylate terpolymers in which some or all of the acid groups are neutralized with metal cations. Such ionomers are commercially available under trademarks such as SURLYN®, which are resins sold commercially by DuPont, of Wilmington, Del., or IOTEK® which is sold commercially by ExxonMobil, of Irving, Tex.
The combination of the above-described core and cover materials provides a “hard” covered ball that is resistant to cutting and other damage caused by striking the ball with a golf club. Further, such a combination imparts a high initial velocity to the ball, which results in increased distance. Due to their hardness, however, these two-piece balls have a relatively low spin rate, which makes them difficult to control, particularly on relatively short approach shots. As such, these balls generally are considered to be “distance” balls. Because the materials of two-piece balls are very rigid, the balls typically have a hard “feel” when struck by a club. Softer cover materials, e.g., balata or softer ionomers or polyurethanes in some instances, have been employed in two-piece balls in order to provide improved “feel” and increased spin rates, although sometimes with a reduction the ball's speed or Coefficient of Restitution (COR).
Regardless of the form of the golf ball, players generally seek a ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, golf ball manufacturers strive to produce balls delivering initial velocities in the U.S.G.A. test that approximate the U.S.G.A. maximum of 77.7 m/s, or 255 ft/s, as closely as possible. Golf ball manufacturers also generally strive to maximize the ball's COR without violating the velocity limitation. Also, to maximize distance, it is advantageous if the balls have a lower driver spin rate. Finally it is highly desirable if, while providing increased velocity and distance, the balls also will exhibit a soft shot feel.
Recently, several golf ball manufacturers have introduced multi-layer balls, i.e., balls having at least a core, an intermediate layer or mantle, and one or more cover layers. The goal of these manufacturers has been to overcome some of the undesirable aspects of conventional two-piece balls, e.g., their hard feel. Such a multi-layer structure allows the introduction of new materials of varying hardness, whereby deficiencies in a property in one layer can be mitigated by the introduction of a different material in another layer. For example, to optimize ball hardness and “feel,” blends of copolymeric high-acid ionomers with softer terpolymeric ionomers have been used as a layer material in a golf ball but again, often with a concurrent loss of COR and/or speed.
Numerous examples of multi-layer combinations are available. For example, U.S. Pat. No. 4,431,193 discloses a golf ball having a multi-layer cover, in which the inner cover layer is a relatively hard, high flexural modulus ionomer resin and the outer cover layer is a relatively soft, low flexural modulus ionomer resin.
Also, U.S. Pat. No. 6,368,237 discloses a multi-layer golf ball comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a high-acid ionomer or ionomer blend. The outer cover layer comprises a soft, very low-modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,416,424, 6,416,424, and 6,419,594, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, very low modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,503,156 and 6,506,130, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, non-ionomeric thermoplastic or thermosetting elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layered golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
Although the use of ionomer(s) in golf balls has found success, it is desirable to develop alternative materials that have similar or superior properties compared to ionomer(s). For example, blending a rigid high-acid ionomer with an elastomer or softer terpolymeric ionomers can improve hardness, but at the expense of diminished COR performance. It would be useful to have a material that could be blended with an ionomer to improve hardness without a deleterious effect on COR.